RCD ELCB RCBO RCCB Electric Shock Protection Explained

In the realm of electrical safety, various devices provide protection against electric shock and fire hazards.

Among these are ELCBs, RCDs, RCBOs, and RCCBs.

Each has distinct functionalities and applications, and they are governed by specific British Standards. Let’s delve into each of these devices:

What is an ELCB (Earth Leakage Circuit Breaker)?

An Earth Leakage Circuit Breaker (ELCB) is a safety device used in electrical installations to prevent electric shocks and protect against fire hazards caused by earth leakages or faults.

While its usage has diminished in the UK, being largely replaced by Residual Current Devices (RCDs), understanding how ELCBs work provides insight into the evolution of electrical safety practices. Plus they are still something that electrical engineers come across every day during electrical safety inspections.

How does an ELCB Works

  1. Detecting Earth Leakage: The primary function of an ELCB is to monitor the current balance between live (hot) wires and the earth. In a normal, fault-free circuit, the current returning through the neutral wire should be equal to the current in the live wire. If there’s an earth leakage, some of the current will flow directly to the earth instead of returning through the neutral wire, creating an imbalance.
  2. Tripping Mechanism: When the ELCB detects this imbalance, indicating a leakage to earth (which could happen if someone touches a live part and provides a path to earth), it triggers a tripping mechanism. This action disconnects the electrical circuit, cutting off the power supply and thus preventing the risk of electric shock or fire.

What are the types of ELCBs

There are two main types of ELCBs:

  1. Voltage-Operated ELCB: This type detects a voltage on the earth wire; when this voltage exceeds a set level, it indicates an earth fault and trips the circuit. Voltage-operated ELCBs were more common in older installations.
  2. Current-Operated ELCB: This type, which is essentially an early form of RCD, monitors the difference in current between the live and neutral wires. Any discrepancy indicating earth leakage will cause the unit to trip.

Are ELCBs still used in the UK?

In the UK, the Earth Leakage Circuit Breaker is obsolete and no longer in use, most have been replaced with RCDs.

RCDs offer several advantages over ELCBs, the newer technology is much more sensitive and detects imbalances more accurately. Like many things, advances in technology mean advances in electrical safety.

Where would I find an ELCB?

Earth Leakage Circuit Breakers were fitted in electrical systems between the 1950’s and 1970s. Most have now been replaced with RCDs, but electrical inspectors need to be aware of the technology as there are still plenty in use in older electrical installations.

Which standards do ELCBs comply to?

Earth Leakage Circuit Breakers were built to comply with IEC 61008, it is an international standard that covers the performance, testing and labelling requirements of both RCD’s and ELCBs.

RCD (Residual Current Device)

An RCD, or Residual Current Device, is a critical safety device in electrical systems designed to prevent electric shock, which can be life-threatening. It offers a high level of personal protection by detecting and responding to electrical faults that could pose a risk of shock.

Let’s explore how a Residual Current Device works and provides this protection:

How does an RCD work?

  1. Detecting Imbalance: The fundamental principle of an RCD is to monitor the electric current flowing through one or more circuits. It constantly checks the balance between the current flowing into the circuit (live wire) and the current flowing out (neutral wire).
  2. Responding to Faults: In a properly functioning circuit, the current flowing in and out should be equal. If a person accidentally touches a live part (like a bare wire) or a fault occurs within an appliance, it can cause some of the current to flow through the person to the ground (earth), creating an imbalance.
  3. Tripping Mechanism: Once the RCD detects this imbalance, even a very small one (typically 5-30 milliamperes), it quickly cuts off the electricity supply to the circuit. This action occurs within milliseconds, often fast enough to prevent a harmful electric shock.

Electric Shock Protection

  • Direct Contact: If someone touches a live part (due to insulation failure, for instance), the body becomes a path for the electrical current to reach the earth. This can result in an electric shock. An RCD can detect this abnormal current flow and disconnect the power before the shock becomes dangerous.
  • Indirect Contact: An RCD also protects against indirect contact, where electricity might flow through a conductive part that has become live due to a fault (like a metal appliance casing). The RCD senses the difference in current caused by the fault and trips the circuit.

What are the advantages of using RCDs?

  1. Fast Response: RCDs react quickly to electricity leaks, significantly reducing the risk of injury or death from electric shock.
  2. Additional Protection: They provide a level of personal protection that ordinary fuses and circuit breakers cannot. While fuses and circuit breakers protect against overloads and short circuits, RCDs specifically protect against earth leakage, which can be a source of electric shocks.
  3. Versatility: RCDs can be used in a variety of settings, including domestic, commercial, and industrial environments. They are particularly important in high-risk areas like bathrooms, kitchens, and outdoor circuits.

Where should we use RCDs?

Over the years the number of RCD’s in use in a property have increased, due to size, costs and improvements in safety standards.

The most common RCD most people see and use is the plug adaptor which is often used when using electric hedge cutters or mowers outside of the equipotential zone. This provides additional electric shock protection to the user.

RCDs have been placed as part of the consumer unit and distribution board equipment formally since 1992. Initially a whole installation RCD or ELCB would be used specifically for TT installations, over time the requirements for RCD protection have increased as we have gained a better technical understanding and standards of electrical safety have improved.

In the 16th Edition Wiring Regulations, split-load consumer units were introduced to domestic premises, with upstairs and downstairs lighting and power being spread between alternate RCDs. This tied up with changes to Part M of the Building Regulations and improvements in accessibility and disability regulations.

As technology has improved and standards raised further, we now the requirements for RCD protection for cables embedded in the walls* and are able to have RCD protection in office and workplace environments.

Relevant BS Standard

BS 7071 covers Residual Current Devices for socket-outlets, and BS EN 61008 and BS EN 61009 cover RCDs in consumer units.

RCBO (Residual Current Breaker with Overload protection)

An RCBO, or Residual Current Breaker with Overcurrent protection, is a compact and efficient device used in electrical circuits for ensuring safety.

It combines the functions of two important safety devices: a Residual Current Device (RCD) and a Miniature Circuit Breaker (MCB).

How does an RCBO work?

  1. Dual Protection: An RCBO provides both earth fault protection (like an RCD) and overload and short circuit protection (like an MCB).
  2. Residual Current Detection: It continuously monitors the current balance between the live and neutral wires in a circuit. If it detects a mismatch, indicating a leakage current (potentially hazardous to humans), it trips and disconnects the circuit, similar to an RCD’s function.
  3. Overcurrent Protection: The RCBO also monitors the current level in the circuit to protect against overload and short circuits. If the current exceeds the rated capacity of the circuit (due to an overload or short circuit), the RCBO trips to prevent wire overheating and potential fire hazards.

In summary, while both RCBOs and Residual Current Devices are crucial for electrical safety, providing protection against potentially dangerous earth leakage currents, RCBOs offer the added advantage of protecting against overcurrents as well.

The choice between an RCBO and an Residual Current Device will depend on the specific requirements of the electrical installation, including the level of protection needed and space considerations in the consumer unit.

Usage and Location

RCBOs are particularly useful in situations where there is limited space in the consumer unit, as they allow for both overload and earth fault protection in one unit. They are commonly found in modern consumer units.

Relevant BS Standards

BS EN 61009-1 specifies requirements for RCBOs.

RCCB (Residual Current Circuit Breaker)

An RCCB, or Residual Current Circuit Breaker, is a safety device used in electrical systems to prevent risks of electric shocks and injuries that can be caused by earth faults.

Here’s a brief explanation of how an RCCB works:

Functioning of an RCCB

  1. Monitoring Current Balance: The core function of an RCCB is to continuously monitor the current balance between the live (phase) and neutral wires in an electrical circuit. In a normal, fault-free situation, the current flowing through these wires is equal.
  2. Detecting Earth Leakage: If there is an earth fault in the circuit (like if someone accidentally touches a live wire), some of the current will leak to the ground instead of returning through the neutral wire. This creates an imbalance in the circuit.
  3. Tripping Mechanism: The RCCB is designed to quickly respond to this imbalance. When it detects a difference between the outgoing and incoming current – typically as low as 30 milliamperes – it trips, automatically cutting off the electricity supply to the affected circuit. This disconnection happens within milliseconds, significantly reducing the risk of electric shock.

How RCCBs Differ from Other Circuit Breakers

Unlike MCBs (Miniature Circuit Breakers) which protect against overloads and short circuits, or RCBOs which combine the functions of RCDs and MCBs, RCCBs solely focus on detecting and preventing earth leakage currents.

They do not offer protection against circuit overloads or short circuits.

Applications of RCCBs

RCCBs are widely used in residential, commercial, and industrial electrical installations, particularly in circuits where there is a high risk of electric shock due to earth faults. This includes areas with higher moisture levels like bathrooms, kitchens, and outdoor settings.

  • Function: An RCCB, similar to an RCD, is designed to protect against the risks of electrocution and fire caused by earth faults. However, unlike an RCBO, it does not offer protection against overload.
  • Usage and Location: RCCBs are suitable where separate overcurrent protection is already provided or not required. They are often used in distribution boards in both residential and commercial settings.
  • Relevant BS Standard: BS EN 61008-1 covers the requirements for RCCBs.
  • Manufacturers: Crabtree, GE Power, and Schneider Electric.

Comparison and Applications

  • ELCB vs. RCD: ELCBs are largely outdated and have been replaced by RCDs due to their more sensitive and reliable protection against earth faults.
  • RCDs and RCCBs: Both provide earth fault protection but RCCBs lack overcurrent protection which is present in RCDs.
  • RCBOs: Offer a compact solution by combining the functionalities of RCDs and MCBs, ideal for consumer units with limited space.

Nuisance Tripping

Nuisance tripping, particularly in setups with multiple Residual Current Devices (RCDs) or banks of RCDs, can be a significant issue in both residential and commercial electrical systems.

This unintended tripping of Residual Current Devices, often at inconvenient times, can be triggered by several factors.

Here are some examples and scenarios where nuisance tripping can occur due to banks of Residual Current Devices:

1. Electrical Noise and Transients

  • In environments with multiple RCDs, electrical noise or transient surges can cause nuisance tripping. This noise can originate from a variety of sources like switching of large inductive loads (e.g., motors starting up), lightning strikes, or power line disturbances. Each RCD may respond differently to these transients, leading to random tripping of one or more RCDs in the bank.

2. High Leakage Currents from Multiple Devices

  • When numerous devices, each with a small amount of earth leakage current, are connected to a circuit protected by an RCD, their combined leakage can sometimes reach the tripping threshold of the RCD. This is more likely in scenarios where a single RCD protects several circuits with multiple appliances.

3. Capacitive Coupling in Long Cable Runs

  • In installations with long cable runs or numerous parallel cables, capacitive coupling can occur. This can induce small leakage currents which may cumulatively trigger a Residual Current Device, especially in banks where several Residual Current Devices are present, and circuits are interconnected.

4. Faulty or Aging Appliances

  • In a bank of RCDs, if one circuit is connected to a faulty or aging appliance that intermittently leaks current to earth, it can cause repeated and unpredictable tripping. This is compounded when there are multiple appliances with minor faults across several circuits, each protected by an RCD.

5. Unbalanced Load Distribution

  • When RCDs are used in banks, an unbalanced load distribution across the circuits can contribute to nuisance tripping. Some RCDs might be under more load than others, making them more susceptible to tripping due to minor fluctuations or disturbances in the system.

6. Incorrectly Rated or Oversensitive RCDs

  • If RCDs in a bank are not correctly rated for the circuits they protect, or if they are overly sensitive, they can trip unnecessarily. This is particularly problematic in complex installations where different circuits have varying load characteristics.

7. Harmonics in Electrical Systems

  • In commercial settings with equipment that generates harmonics (like variable speed drives, electronic lighting, and certain types of office equipment), these harmonics can lead to nuisance tripping of RCDs, especially when multiple RCDs are involved.

Mitigating Nuisance Tripping

To reduce the likelihood of nuisance tripping in systems with banks of Residual Current Devices, it’s important to:

  • Ensure proper load distribution and circuit separation.
  • Regularly maintain and check the electrical installation and connected appliances for faults.
  • Use RCDs with appropriate sensitivity and ratings for the specific loads and conditions of each circuit.
  • Consider using RCDs with higher immunity to transients in environments prone to electrical noise.

Selection of RCDs for Additional Protection

In line with BS 7671 standards for electrical installations in the UK, Residual Current Devices (RCDs) with different tripping currents are used to cater to varying levels of safety requirements.

Here’s a simplified overview:

30mA Residual Current Devices: Primarily used for enhanced protection against electric shock in residential settings, especially in areas like bathrooms and kitchens, or for outdoor socket outlets. BS 7671 recommends these for their high level of personal safety, particularly in environments where electrical equipment might be used outdoors.

100mA, 300mA, and 500mA Residual Current Devices: These higher-rated RCDs are generally used in commercial or industrial settings. The 100mA RCDs strike a balance between protecting against electric shock and minimizing nuisance tripping in larger installations. On the other hand, 300mA and 500mA RCDs focus more on fire prevention and protecting equipment rather than direct personal protection. They are typically employed in main distribution boards or for high-level distribution circuits.

Understanding the Types of Residual Current Device

Residual Current Devices (RCDs) come in different types – AC, A, F, and B – each designed to respond to different types of residual currents. Understanding their differences is crucial for ensuring the right protection in various electrical installations.

Type AC Residual Current Devices are designed to detect and respond to alternating sinusoidal currents only. They are suitable for protecting equipment and circuits that have purely resistive loads and do not produce any DC components or pulsating currents. Type A Residual Current Devices, on the other hand, can detect both alternating currents and pulsating direct currents. This makes them more versatile and suitable for modern households and commercial environments where electronics, such as computers and modern appliances that can generate rectified currents, are common.

Type F and Type B Residual Current Devices represent more specialised devices.

Type F Residual Current Devices are designed to work with frequency-controlled devices, suitable for modern appliances with variable speed drives, like energy-efficient washing machines and some types of air conditioning units.

Type B RCDs are the most advanced, capable of detecting all types of residual currents including smooth direct currents. They are essential in installations with electric vehicle (EV) charging points and equipment that generate high levels of DC currents. The presence of such modern equipment in electrical installations necessitates the use of Type B RCDs to ensure comprehensive protection against both AC and DC residual currents, which Type AC RCDs cannot detect.

This highlights the importance of selecting the appropriate Residual Current Devices type based on the specific characteristics of the electrical equipment and devices in use.

Testing of RCDs, RCBOs, ELCBs and RCCBs to BS 7671

Guidance Note 3 of BS 7671 provides detailed procedures for the inspection and testing of electrical installations, including the testing of Residual Current Devices.

The testing of Residual Current Devices is crucial to ensure they are operating correctly and providing the necessary protection.

Here’s an outline of the recommended test procedures for Residual Current Devices according to this guidance:

1. Visual Inspection

  • Before any testing, visually inspect the device for any signs of physical damage, correct rating for the circuit it’s protecting, and proper labeling.

2. RCD Trip Time Test at Rated Tripping Current

  • This test involves measuring the time the Residual Current Device takes to trip at its rated tripping current. Use an RCD tester to apply a fault current equal to the rated tripping current of the device (usually 30 mA for general use). The device should trip within 300 ms for general purpose RCDs or within 40 ms for RCDs protecting socket outlets that may be used to supply portable equipment outdoors.

3. RCD Trip Time Test at 5 Times Rated Tripping Current

  • This test is similar to the previous one but involves applying a fault current of 5 times the rated tripping current. This ensures that the Residual Current Device trips quickly under higher fault conditions. For a 30 mA Residual Current Device, this would be a 150 mA fault current. The Residual Current Device should trip within 40 ms.

4. RCD Trip Current Test (Ramp Test)

  • The purpose of this test is to determine the minimum current at which the Residual Current Device will trip. It gradually increases the fault current until the Residual Current Device trips. This test is useful for verifying that the Residual Current Device is not overly sensitive (tripping at currents significantly lower than its rated sensitivity) or insufficiently sensitive (not tripping at its rated sensitivity).

5. Testing for All Possible Combinations of Live and Neutral Connections

  • This involves repeating the tests with different combinations of connections to live and neutral conductors. This ensures that the Residual Current Device trips regardless of the direction of the fault current.

6. Half-Wave Rectified DC Test

  • Some RCDs may fail to trip in response to pulsating DC fault currents. This test, using a half-wave rectified DC current, checks the RCD’s response to such conditions.

7. Functional Test

  • This is a simple test where you press the ‘Test’ button on the Residual Current Device to ensure it trips and resets correctly. It should be done at regular intervals, as recommended by the manufacturer, usually quarterly.

It’s important to note that these tests should be carried out by a competent person, typically an electrician or a testing technician, using the appropriate calibrated test equipment, such as an RCD tester.

Accurate testing ensures that the Residual Current Devices are reliable and effective in providing protection against electric shock and other hazards associated with earth faults. Regular testing, as outlined in Guidance Note 3 of BS 7671, is essential for maintaining the safety and compliance of electrical installations.


ELCBs, RCDs, RCBOs, and RCCBs are all critical components of modern electrical safety systems, each serving a specific purpose.

The choice between these devices depends on the specific requirements of the electrical installation, including space constraints, the need for combined overcurrent and earth fault protection, and the level of safety required.

Adhering to the relevant British Standards ensures compliance with safety regulations and optimal protection in electrical installations.

Manufacturers like Schneider Electric, ABB, Eaton, and Legrand offer a range of these devices, catering to various safety needs in the electrical industry.